The refractive index profile of optical fibers is generally qualified in relation to the plotting of a graph that shows the function associating the refractive index of the fiber with the radius of the fiber. Conventionally, the distance, r, to the center of the fiber is shown along the abscissa axis, and the difference between the refractive index of the fiber core and the refractive index of the fiber cladding is shown along the ordinate axis. The index profile is therefore described as “step,” “trapezoid,” or “triangular” for graphs, showing, respectively, step, trapezoid, or triangular shapes. These curves are generally representative of the theoretical profile (or set profile) of the fiber. The manufacturing of the fiber may lead to a slightly different profile.
An optical fiber conventionally consists of an optical core whose function is to transmit and optionally amplify an optical signal and an optical cladding whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core, nc, and the cladding, ng, are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber breaks down into a fundamental mode guided in the core and secondary modes guided over a certain distance in the core-cladding assembly (i.e., cladding modes).
The signal transmitted in the fiber undergoes optical losses that accumulate over the distance traveled. These transmission losses increase when the fiber is subjected to ionizing radiation, such as beta, alpha, gamma and X radiation. In this regard, the fiber may be subjected to radiation when it is used for an optical communications system in an environment having ionizing radiation (e.g., in a nuclear plant, a particle acceleration laboratory, or a satellite sent into space). In such environments, radiation may reach dose levels of 100 gray (Gy) or more (i.e., 10,000 rad or more).
Conventionally, Standard Single Mode Fibers (SSMF) are used as line fibers in optical fiber transmission systems. The step index is generally obtained by doping with phosphorus or germanium, which can increase the refractive index. The step index may also be obtained by doping with fluorine, which can reduce the refractive index. These optical fibers (SSMF) have low transmission losses, generally less than 0.4 dB/km over a broad spectral range (at least 1300-1650 nanometers) but are sensitive to ionizing radiation. A SSMF fiber placed in an environment radiating 30 kGy will have its transmission losses increased by 10 to 10,000 dB/km at a wavelength of 1310 nanometers. This increase strongly depends on radiation conditions, particularly dose rate. A conventional SSMF fiber, therefore, is not suitable for use in an optical communications system installed in an environment having high doses of ionizing radiation (e.g., higher than 100 to 1,000 Gy).
Some known fibers are specifically designed for use in an environment where there is ionizing radiation. For example, U.S. Pat. No. 4,690,504 discloses a single-mode optical fiber with a germanium-free core. The absence of germanium in the core makes it possible to obtain better resistance to ionizing radiation. The optical cladding is doped with a dopant, such as fluorine, that reduces the refractive index. This patent also discloses an embodiment in which the core of a fiber is slightly fluorine-doped to offset excess oxygen in the core.
U.S. Pat. No. 5,509,101 discloses an optical fiber resisting X and gamma radiation. This fiber has a core and a cladding doped with fluorine. This patent describes several embodiments with different concentrations of fluorine and germanium. This patent indicates that transmission losses are reduced when the core of the fiber also includes germanium.
International Publication No. WO 2005/109055 discloses an optical fiber with a core of pure silica and a fluorine-doped cladding. This publication indicates that a high ratio (e.g., 9-10) between the diameter of the cladding and the diameter of the core improves the fiber's resistance to ionizing radiation.
The prior art fibers show some resistance to ionizing radiation but nevertheless have high losses under strong radiation, particularly over and above 400 Gy. As noted, standard single mode fibers (SSMFs) are typically used as line fibers in optical fiber transmission systems. Such fibers have a chromatic dispersion and a chromatic dispersion slope meeting specific telecommunications standards. To promote compatibility between the optical systems of different manufacturers, the International Telecommunication Union (ITU) has established a standard referenced ITU-T G.652, which must be met by a Standard Single Mode Fiber (SSMF). This standard is sub-divided into four sub-standards (A, B, C, and D) of greater or lesser severity.
For example, the G.652B standard for transmission fibers, recommends inter alia: a range of between 8.6 and 9.5 microns for the Mode Field Diameter (MFD) at a wavelength of 1310 nanometers; a maximum of 1260 nanometers for the cabled cut-off wavelength; a range of between 1300 and 1324 nanometers for the zero dispersion wavelength (λ0); and a maximum of 0.093 ps/nm2-km for the chromatic dispersion slope value.
The cabled cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single-mode after propagation over 22 meters of fiber, such as defined by sub-committee 86A of the International Electrotechnical Commission under standard IEC 60793-1-44.
Despite prior efforts, there remains a need for a transmission fiber that shows an improved resistance to high-dose radiation.